1. Field of the Invention
The present invention relates to a method of tracing a conductor e.g. the route of pipes and cables; and to an apparatus therefore. By cable we include electric cables, optical fiber cables (where the conductive armouring provides a conductive path). Indeed the invention is applicable to any metallic pipe or duct. It is now well known to induce a signal into a metallic conductor, and to detect the propagation of that signal at a remote point, to detect the conductor""s position or faults in the conductor. This technique is particularly, but not exclusively, applicable where the conductor (such as an electric cable) is inaccessible, or hidden e.g. underground.
2. Summary of the Prior Art
Many techniques have been developed for analysing the signals, which propagate down the conductor, and various techniques have been established for remotely detecting those signals. In general, the field produced by the alternating current on the service may be sampled using an aerial placed in the vicinity of the conductor; the relative magnitude of the detected signal at various points conveys information about the conductor""s position. One particular configuration uses one or more horizontal coils so that the detected signal is a maximum when the coil is directly above and orthogonal to the conductor. The phase of this signal with respect of the transmitted signal will be constant at a point, the value determined by the electrical properties of the circuit.
However, much of the theory of such detection has been determined on the basis that there is a single conductor. In practice, such conductors are usually in-groups. The problem then is that, although it is possible to ensure that the transmitter induces current only into one conductor, nevertheless there is likely to be capacitative coupling or even direct bonding between the conductors along their length, and therefore currents will also be generated in the other conductors. At first sight, these other currents should be of much lower amplitude, but in fact the detected signal, derived from the magnetic flux seen by the receiving aerial may not differ significantly between one conductor and another. Bearing in mind that the detection of the signal may have to be from a point remote from the conductor, the distance between the detector and the conductor may therefore be a significant factor in the magnitude of the signal detected. For example, if the conductor to which the current is actually applied is somewhat further away from the detector than another conductor to which current has capacitively been transmitted, then the signal from the latter conductor may be of comparable magnitude with that in the conductor to which the current has been applied. As a result, it is impossible to tell the two apart, and thus the measurement is ineffective.
U.S. Pat. No. 5,260,059 (the disclosure of which is incorporated herein by reference) sought to solve this problem by applying a signal current for transmission which comprised at least two components related in frequency and phase. The frequency relation was one based on a direct harmonic, with one component being an integer (normally an even integer) multiple of the other. However, it was also maintained that component may be produced by combining a sub-harmonic of the frequency of the other with that frequency.
When the signal was capacitively transmitted to another conductor, the phase relationship of the two signals was reversed. Therefore, by comparing the phase of the signal determined at one point with the phase of the signal at another point, it became possible to determine the conductor to which the signal had actually been applied. The present invention seeks to develop the ideas of U.S. Pat. No. 5,260,659. A disadvantage of using two components which are harmonics of each other is that such harmonics may occur naturally, in arrangements where the applied signal was only one component and therefore the arrangement of U.S. Pat. No. 5,260,659 is not wholly reliable.
Therefore, the present invention proposes that the frequencies F1 and F2 of the two components be related by Nxc3x97F1=Mxc3x97F2where N and M are non-adjacent integers greater than 1, one of which is odd and one of which is even, with N and M having no common factors.
Then, as in U.S. Pat. No. 5,260,659 one of the points at which the signal is determined may be the point of transmission, so that an absolute value is used. Alternatively it was possible to measure at one point along the cable or pipe, and then measure again at a further point, with the first measurement as a reference, the difference in phase between that first point and the second point could be investigated, i.e. it is a relative measurement. The advantage of this is that comparison of the detected signal with the transmitted signal depends on there being negligible phase shift of the signal along the cable. However, particularly at high frequencies, this phase shift may not be negligible due to the resistance and capacitance effects of the pipe or cable, and therefore a relative system must be used.
To investigate the phase relationship, several different analysis methods may be used.